1. Field of the Invention
The present invention relates to a read/write position controlling apparatus of an optical pickup which performs read/write operation from/to a recording medium.
2. Description of the Related Art
Research and development of high-density large-capacity information recording media is now actively pursued with the recent development of an information and communications technology. Such recoding media include optical recording media and magnetic recording media. For example, known optical recording media include optical discs such as a compact disc (CD) and a digital versatile disc (DVD). Also it is known to provide a multilayer optical disc capable of increasing the recording capacity of each recoding surface by providing a plurality of recording layers on the same recording surface (side) for larger capacity optical discs.
Furthermore, research and development of an optical disc that uses a blue violet laser as a light source, for example, Blu-ray Disc, is energetically pushed forward as a next-generation large-capacity optical disc.
It is required to increase the performance of read/write device with increasing density of the recording media. Particularly, it is important to achieve high performance of read/write position controllers including servo control for controlling the read/write position on the recording media.
A position controller using nonlinear control (sliding-mode control) has been proposed. For example, refer to Japanese Patent Application Kokai H10-106012. The controller is used for controlling a recording-medium (recording-paper) feeding system of a printer and so on, using sliding-mode control in order to prevent remaining vibration immediately before the stop of feeding of the recording medium to a target position. Upon stopping the feeding of the recording medium, the target position control is also stopped.
The nonlinear control is disadvantageous in that control input tends to produce chattering (high-frequency vibration) and power consumption is large.
As set forth hereinabove, it is important to increase the performance of the controller used for recording media such as recording discs capable of writing information and reading the written or recorded information with the development of high-density recording. For example, reader/writers that use optical recording discs need to move and focus a light beam used for writing and reading to a target write or read position and thereafter to perform writing or reading operation while performing tracking control and focusing control. Thus, it is required to enhance the speed, accuracy, and stability of the pickup write/read position controller, including servo control such as tracking control and focusing control.
As described above, it is necessary for the high-recording-density recording media to move light beams to the vicinity of a target position (perform a tracking jump or a focus jump) with the servo is open and then to perform a servo pulling-in operation quickly and stably. Furthermore, the high-speed and stable servo pulling-in is necessary also after an actuator is driven for preventing unstable servo control due to the defects of the recording media.
For example, optical discs generally have various defects, such as scratches, dirt, and finger prints, formed when the discs are manufactured or used, which cause impediments to stable servo control. The existence of the defects causes incorrect indications of a tracking error and a focus error, disturbing the servo control when the actuator is driven in accordance with the indications.
Conventional servo control has employed linear control in which an error signal is subjected to linear phase compensation and then the actuator is driven. In order to prevent adverse effects on the servo control due to the defects of the disc, a method has been employed in which when a defect is detected, servo failure due to a false error signal (i.e., not indicating a correct amount of deviation from a target value) is reduced by using an error value (“former value” hold) before the detection of the defect or the low-frequency component (or, integral signal) of the error signal as a servo control signal. For example, refer to Japanese Patent Application Kokai No. 2002-008253.
Even such defect detection may cause time delay (detection delay time or, simply, delay time) from the occurrence of a defect until the detection of the defect. During the delay time, servo control is performed on the basis of the false error signal affected by the defect, causing the disturbance of servo control. For example, refer to Japanese Patent Application Kokai No. 2002-008253. Also the operation of changing the characteristics of a liner controller before and after the defect detection has been performed. For example, refer to Japanese Patent Application Kokai No. 2001-307346.
The above-described related-art servo control using the hold-value is, however, disadvantageous in that the deviation amount indicated by the hold-value and an actual deviation amount are greatly different from each other during the occurrence of a defect, causing a transient phenomenon after completion of the defect to delay the convergence of the error signal to a normal value, thus making the servo control unstable. Also, there is a limitation to improvement of the convergence by changing the characteristics of the linear controller after completion of the defect, which is insufficient. Furthermore, for example, when a defect such as a finger print appears repeatedly, the interval between the defects is extremely short, thus making it difficult to pull-in the servo stably within the valid section therebetween. For example, when a defect appears repeatedly, with only normal linear control, the servo control is disturbed by the following defect before the pulling-in operation is surely performed after completion of one defect. The repetition of the disturbance causes deviation to be gradually accumulated to be increased, thus making it difficult to perform stable servo control.